As is well known in the art, field-access operation of magnetic bubble memories employs a magnetic field which reorients, typically by rotation, in the plane in which bubble movement occurs. The rotating field generally is provided by two field coils oriented orthogonally to one another and physically encompassing the plane of movement. The coils are driven in quadrature by a pair of tuned circuits as described, for example, in copending application Ser. No. 443,960 filed Feb. 20, 1974, now U.S. Pat. No. 3,879,585, issued Apr. 22, 1975.
Bubble memories are known to be nonvolatile. That is to say, bubble memories retain information even when a power failure occurs. It is important, however, to ensure that the in-plane field stops and starts in the same orientation in order to avoid loss of information. Consequently, circuits generating the in-plane fields have required precise adjustment to ensure that they were properly energized and de-energized.
To be specific, information can be lost if energization of the bubble circuit does not occur at the precise point in the in-plane field cycle at which de-energization previously occurred. Loss occurs because when energization occurs, bubbles might be at positions with respect to the permalloy pattern to not move at all or to move incorrectly. Since the in-plane field is supplied by two coils driven in quadrature and since the coils constitute parts of two tuned circuits, a variation in the peak currents in the coils and/or the resonant frequencies of the tuned circuits causes the in-plane field to deviate from a constant amplitude, which deviation could result in a change in the phase relationship of the energization and de-energization signals thus increasing the risk of information loss.
In order to maintain peak currents in the two coils at constant amplitude, refresh pulses normally are applied to the tuned circuits during each cycle of the in-plane field. The refresh pulses, hitherto, have been supplied by an external clock which, because of the above considerations, itself had to be precisely set with respect to the phase of the in-plane field. Moreover, the tuned circuits were still subject to variations in their voltages which changed the phase relationship not only between the two tuned circuits but also between the two circuits and the clock. Consequently, considerable care (and expense) has been taken to supply properly timed refresh pulses and to avoid such variations.
During energization and de-energization, the shape of the in-plane field also is important. Ideally, the components of the in-plane field do not change from zero to full field strength simultaneously during energization but rather one component initiates its rise at the precise time the other component reaches its peak. During de-energization, a similar timing relationship also holds true. If the timed sequence is not observed, the resulting non-ideal build-up or decay of the field could, again, result in undesirable movement of bubbles and thus loss of information. The achievement of a suitably timed sequence of field component changes, properly timed refresh pulses, and the avoidance of voltage drift in the tuned circuits has required sophisticated control circuitry for bubble memory operation.